Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of delivering power economically and with environmental and other benefits. To be commercially viable, however, fuel cell systems should exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside their preferred operating ranges.
Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
A broad range of reactants can be used in solid polymer electrolyte fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode electrocatalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode electrocatalyst. The electrocatalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (“MEA”), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
Each electrode contains an electrocatalyst layer, comprising an appropriate catalyst for facilitating the desired electrochemical reaction of the fuel and oxidant, located adjacent the solid polymer electrolyte. The electrocatalyst may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The catalyst layer typically contains ionomer that may be similar to that used for the solid polymer electrolyte (for example, Nafion®). The catalyst layer may also contain a binder, such as polytetrafluoroethylene.
The electrodes may also contain a substrate (typically a porous electrically conductive sheet material) that may be employed for purposes of reactant distribution and/or mechanical support. Optionally, the electrodes may also contain a sublayer (typically containing an electrically conductive particulate material, for example carbon black) between the catalyst layer and the substrate. A sublayer may be used to modify certain properties of the electrode (for example, interface resistance between the catalyst layer and the substrate).
Electrodes for a MEA can be prepared by first applying a sublayer, if desired, to a suitable substrate, and then applying the catalyst layer onto the sublayer. These layers can be applied in the form of slurries or inks that contain particulates and dissolved solids mixed in a suitable liquid carrier. The liquid carrier is then evaporated off to leave a layer of particulates and dispersed solids. Cathode and anode electrodes may then be bonded to opposite sides of the membrane electrolyte via application of heat and/or pressure, or by other methods. Alternatively, catalyst layers may first be applied to the membrane electrolyte with optional sublayers and substrates incorporated thereafter, either on the catalyzed membrane or an electrode substrate.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force effects sealing and provides adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
Electrochemical fuel cells are occasionally subjected to a voltage reversal condition, which is a situation in which the cell is forced to the opposite polarity. Opposite polarity may be deliberately induced, as in the case of certain electrochemical devices known as regenerative fuel cells. (Regenerative fuel cells are designed and constructed to operate both as fuel cells and as electrolyzers in order to produce a supply of reactants for fuel cell operation. Such devices have the capability of directing a water fluid stream to an electrode where, upon passage of an electric current, oxygen is formed. Hydrogen is formed at the other electrode.) Power-producing electrochemical fuel cells connected in series are potentially subject to unwanted voltage reversals, however, such as when one of the cells is forced to the opposite polarity by the other cells in the series. In fuel cell stacks, this can occur when a cell is unable to produce, from the electrochemical reactions occurring within it, the current being produced by the rest of the cells in the stack and that is being directed through the affected cell by virtue of its being in series with the rest of the cells. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the affected fuel cell, and in turn, its associated stack and array.
The adverse effects of voltage reversal can be prevented, for instance, by employing diodes capable of carrying the stack current across each individual fuel cell or by monitoring the voltage of each individual fuel cell and shutting down an affected stack if a low cell voltage is detected. Since stacks typically employ numerous fuel cells, however, such approaches can be quite complex and expensive to implement.
Alternatively, other conditions associated with voltage reversal may be monitored instead, and appropriate corrective action can be taken if reversal conditions or the onset of reversal conditions are detected. For instance, a specially constructed sensor cell may be employed that is more sensitive than other fuel cells in the stack to certain conditions leading to voltage reversal (for example, fuel starvation of the stack). Thus, instead of monitoring every cell in a stack, only the sensor cell need be monitored and used to prevent widespread cell voltage reversal under such conditions. Other conditions leading to voltage reversal may exist that a sensor cell cannot detect, however (such as, for example, a defective individual cell in the stack). Another approach is to employ exhaust gas monitors that detect voltage reversal by detecting the presence of or abnormal amounts of species in an exhaust gas of a fuel cell stack that originate from reactions that occur during reversal. While exhaust gas monitors can detect a reversal condition occurring within any cell in a stack and may suggest the cause of reversal, such monitors do not specifically identify problem cells and they do not generally provide a warning of an impending voltage reversal.
Instead of or in combination with the preceding, a passive approach may be preferred such that, in the event that reversal does occur, the fuel cells are either more tolerant to the reversal or are controlled in such a way that degradation of critical hardware is reduced. A passive approach may be particularly preferred if the conditions leading to reversal are temporary. If the cells can be made more tolerant to voltage reversal, it may not be necessary to detect for reversal and/or shut down the fuel cell system during a temporary reversal period.